Heygster, Georg
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Heygster, Georg
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Heygster, Georg
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Item-typ:Veröffentlichung, Thickness retrieval and emissivity modeling of thin sea ice at L-band for SMOS satellite observations(2015-11-18); ; ; In this study we have developed an empirical retrieval for thickness of young and first-year ice during the freeze up period for the L-band passive microwave radiometer Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) on the Soil Moisture and Ocean Salinity (SMOS) satellite. The retrieval is based on intensity and polarization difference using the incidence angle range of 40° to 50° and is validated using data from airborne EM-Bird, Moderate-resolution Imaging Spectroradiometer (MODIS) thermal imagery, and self consistency checks for ice thicknesses up to 50 cm with an error of 30 % on average. In addition, we modeled the microwave emission for Arctic first-year ice using the sea ice version of the Microwave Emission Model of Layered Snowpacks (MEMLS). The sea ice conditions used as input for MEMLS were generated using a thermodynamic energy balance model (based on the Crocus model) driven by reanalysis data from European Centre for Medium-Range Weather Forecasts (ECMWF). From unexpected features in the modeled microwave emission and disagreements with the empirically trained SMOS retrieval several shortcomings of the energy balance model and MEMLS were identified and corrected. The corrections include a treatment of mismatch of layer definition between the energy balance model and MEMLS, an adaptation of the reflection coefficient for lossy media in MEMLS, and several smaller corrections. For comparison, two simple models ignoring volume scattering, one incoherent and one coherent, were set up and were found to be able to reproduce the results of the more complex MEMLS model on average. With the simple models, the effects of thin coherent layers, the snow cover, the interface roughness and three different dielectric mixture models for sea ice were explored. It was found that the choice of the mixture model is essential for the relation of sea ice thickness to brightness temperatures in L-band, suggesting sea ice thickness sensitivities from few centimeters to several meters for salinity conditions of the global oceans. The interface properties, especially at the sea ice bottom, were found to be a major uncertainty source when modeling the microwave emission of thin sea ice. In addition, the variability in snow depth, the interface roughness, and the ice surface salinity and temperature were found to have a similar influence on the resulting brightness temperatures, with a strong effect on horizontally (up to 30 K) and weak effect on vertically polarized radiation (up to 10 K) for temperatures below 260 K. A model for simulating coherent microwave emission for thickness distributions of ice and snow was prepared to overcome weaknesses from the single thickness coherent and incoherent models. Comparison to the incoherent model showed that for realistic snow depth distributions obtained from Operation IceBridge (OIB) coherence effects can change the brightness temperatures on the scale of a SMOS footprint up to 10 K in horizontal polarization. These findings suggest that the retrieval for the thickness of thin sea ice with satellite based L-band sensors yield higher uncertainties than expected from earlier studies.Dissertation588 200 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Improving an optimal estimation algorithm for surface and atmospheric parameter retrieval using passive microwave data in the Arctic(2018-08-23); ; ; In this study we present improvements on an integrated retrieval method for atmospheric and surface parameters in the Arctic. The instrument used is the Advanced Microwave Scanning Radiometer - Earth Observing System (EOS) (AMSR-E) radiometer on board NASAa s Aqua satellite. The core of the method is a forward model which can ingest bulk data for seven geophysical parameters to reproduce the brightness temperatures observed by a passive microwave radiometer. The method inverts the forward model and produces ensembles of the seven parameters: wind speed, integrated water vapor, liquid water path, sea and ice temperature, sea ice concentration and multi-year ice fraction. The method is constrained using numerical weather prediction data in order to retrieve a set of geophysical parameters that best fit the measurements. An iterative method minimizes the cost function and finds the optimal ensemble of the seven parameters that best match the observed brightness temperatures.Dissertation651 201 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Implementation and validation of the snow grain size retrieval SGSP from spectral reflectances of the satellite sensor MODIS(2011-07-01); ; ; Snow is part of the cryosphere in the climate system of the Earth. It has a high albedo in the visible, decreasing towards the near-infrared. Snow on ground is a porous medium of ice, air, and possibly impurities like dust or soot. After deposition, it undergoes snow metamorphism changing the grain size, grain shape, and density. In the visible, the reflection characteristics of snow are mainly determined by the amount of impurities, and in the near-infrared by the size of the snow grains. Satellite sensors allow observing the snow in remote areas like the polar regions on a regular basis and on a global scale. A method to compute the snow grain size and impurity amount from optical satellite observations is the Snow Grain Size and Pollution amount (SGSP) retrieval. It uses data of three reflectance channels (here: at 0.47 µm, 0.86 µm, and 1.24 µm), has a reduced dependency on the snow grain shape, and is applicable at solar zenith angles up to 75°. In this work, the SGSP retrieval is implemented in a near-real time processing chain using data from the Moderate Resolution Imaging Spectrometer (MODIS) operating on the satellites Terra and Aqua. A sensitivity analysis reveals that currently only the snow grain size can be determined reliably by the SGSP retrieval, as the uncertainties of the MODIS instrument are too high for the amount of impurities typically occurring in polar regions. Sensitivity studies on the influence of vertically inhomogeneous snow, wet snow, and cirrus clouds show that the SGSP retrieval typically underestimates the grain size by 15% to 25% for those three cases. The SGSP-retrieved snow grain size is validated using six different ground truth data sets from the Arctic, the Antarctic, Greenland, and Japan from the years 2001 to 2009, and various subsurfaces (land, land ice, sea ice, lake ice). In general, the retrieved and ground-measured grain size are in good agreement. 17 cases have small differences (1 14%), 16 cases intermediate differences (18 53%), and four cases large differences (72 178%). The SGSP retrieval tends to underestimate the grain size for wet snow cases (by 18% 31%) and cirrus cloud cases (by 14% 31%), and overestimates it for surface hoar cases (by 30% 53%) and wind crust cases (by 23% 77%). A comparison of the SGSP retrieval with a previous retrieval using ground measurements from the Himalayan basin shows that the SGSP-retrieved grain size tends to be smaller (by 5 48 µm) and that vertically inhomogeneous snow influences the retrieval. A comparison of SGSP-retrieved snow grain size time series on the Ross ice shelf, Antarctica, at three Automatic Weather Stations (AWS) with snow depth change data from those three stations shows that a snow fall event of 6 cm is detected by the sudden decrease of the retrieved grain size from 200 µm to 50 µm. A comparison of the spectral snow albedo for the MODIS Channels 1 to 5 over 16 days on a large-scale area in Greenland between the SGSP-derived albedo and the spectral MODIS albedo product MOD43 shows a correlation of 0.82 for Channel 5,which is most sensitive to the snow grain size.Dissertation537 285 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Polarimetric Remote Sensing of Land and Snow/Ice Covers with the SpaceborneMicrowave Radiometer WindSat(2007-06-06); ; ; Measurements from spaceborne microwave radiometers, such as the Scanning Multichannel Microwave Radiometer (SMMR), the Special Sensor Microwave/Imager (SSM/I) and the Advanced Microwave Scanning Radiometer (AMSR), are found to be useful in estimating various earth surface geophysical quantities, e.g. soil moisture and vegetation characteristics over land, snow water equivalent for snow covers and sea ice concentration. All these instruments have measured onlythe vertical and horizontal polarization component of the brightness temperature (Tb ). WindSat is the first spaceborne radiometer to provide fully polarimetric measurements of the earthà ¯Ã ¿Ã ½s emission. It waslaunched by US Navy in February 2003. It determines the polarization state of the emission in the form of Stokes vector consisting of four components. The first two components are the typically measured vertical and horizontal TB. WindSat additionally determines the difference between à ?à ±45 Deg linearly polarized (3rd Stokes component) and left and right hand circular polarized radiation (4th Stokes component). The polarimetric radiometry is primarily used to estimate the sea surface wind speed and direction. So far little was known aboutthe information content of the Stokes vector over vegetation, bare soil, snow and sea ice. This thesis explores the polarimetric signal observed by WindSat over land, the Antarctic ice sheet and Arctic sea ice. Over land, it is shown that the polarimetric signal depends on theorientation of surface features such as sand dunes in deserts, and extended structures in agricultural fields. This dependency is validated over the test sites of the Taklamakan desert and the Helongjiang agriculture fields in China using correlative data collected by the Advanced Spaceborne Thermal Emission and Refection Radiometer(ASTER). The ASTER images from thermal infrared sensor of 90 meter resolution are used to identify the vegetation and desert surface structures. Over the Antarctic ice sheet, the findings show that the polarimetric signatures at higher frequencies (37 GHz) depend on snow surface features, such as sastrugi orientation and surface topography, whereas at the lower frequency (10.7 GHz) the signal additionallydepend on snow properties such as grain size and density.This dependency is validated using the results from previously made scatterometer studies with European Space agency SCATterometer (ESCAT) and NASA SCATterometer (NSCAT), demonstrating the potential of polarimetric radiometers to estimate scattering and emission properties from the single instrument. Finally, the analysis of one year of the weekly averaged 3rd and 4th Stokes components of Arctic sea ice shows the highest anisotropic signal during a week of early summer (June 21-27). This was interpreted as a result of melting of the overlying snow due to which the penetration depth decreases making higher frequencies (37 GHz) sensitive to surface structure.Dissertation578 161 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, On observations of artificial light at night from ground and space(2017-04-27); ; ; To assess the negative effects of artificial light at night, measurement data are often necessary. These can be acquired from ground or satellite-based measurements. Satellite-based observations of artificial light have an advantage of global coverage. Since the launch of the Suomi National Polar-orbiting Partnership (S-NPP) satellite, the Visible Infrared Imaging Radiometer Suite (VIIRS) Day-Night Band (DNB) provides a tool for worldwide night time remote sensing for various applications, including artificial light. We used the data in year 2015 from the VIIRS-DNB sensor to study the angular distribution of artificial light in major cities and metropoles in Europe. Despite encountering some issues, for example low overflight rate with cloud-free sky condition, we are able to investigate the angular distributions of upwelling artificial light emission for 74 regions, almost all of which emit more light near the horizon. In comparison to satellite-based observations, ground-based measurements can acquire data more frequently. An international campaign studying variations of night sky at 44 worldwide locations was conducted in 2011 and 2012, measuring the night sky using broadband radiometers, the Sky Quality Meters (SQMs), and was the first time comparing ground-based night sky measurement in a unified analysis procedure. The presence of artificial light reverses the variation pattern of sky brightness at cloudy nights. Instead of darkening of night sky by blockage of extraterrestrial light sources, clouds backscatter the artificial light emitted from ground, brighting the night sky. Comparison with daytime aerosol measurement data did not yield a consistent relationship between the aerosol content and night sky brightness for the rural site in Cabauw, the Netherlands, but found the brightening of the night sky of Madrid with increasing atmospheric aerosol load. Long-term monitoring of the night sky enables us to observe the change in skyglow pattern over a long period. Two SQMs have been set up in a suburban site and a rural site in Bremen, Germany since December 2011. The night sky of the suburban site is found to be brighter during cloudy nights and becomes darker during the course of the night, which is typical for a light-polluted location. While instrumental issues cannot be ruled out, we found a decrease of sky brightness over a span of about 4.5 years, with a stronger decreasing trend at late hours of the night. For the rural site, the local public and domestic lightings are the dominating light source, contributing to the larger skyglow in the early hours of the night. However, with domestic and public lighting switched off as the night progresses, the variation pattern of the night sky is typical for a location with less light pollution, where the overcast sky is only slightly different in brightness from clear sky. It is suspected that this trend in the change of sky brightness originates from the gradual decrease of output of the public lighting system due to aging, while the progress replacing the public lighting to light-emitting diode (LED) devices is slow. An improvement in terms of ecological impacts in the future is therefore not guaranteed.Dissertation474 161